Laparoscopic instruments having a tungsten carbide shaft have recently been discussed. The advantage of ceramics in comparison with steel, which is generally, used is that ceramics have a much greater strength, in particular a better flexural rigidity, which is a major advantage in laparoscopic instruments having a long, thin shaft, when greater forces are exerted with such instruments.
However, the danger of breakage is a disadvantage in the case of shafts made of ceramic. If the load limits are exceeded, the shaft does not bend, as would be the case with a shaft made of steel, but it breaks instead. However, this could have catastrophic consequences in an operation with such an instrument. If a ceramic shaft breaks, sharp fracture edges are usually formed and could lead to injuries in the patients body. Furthermore, splinters may be formed and can lead to serious problems if they enter the abdominal cavity, especially since such splinters are difficult to locate.
If the ceramic is enclosed by a sheath which covers at least those regions of the ceramic tube that are at risk of breakage. The sheath is also designed with material properties such that it prevents the penetration of broken ceramics; although it does not prevent the ceramic from breaking, the harmful consequences of such a break are prevented. Sharp edges that are formed on the ceramic tube in the event of breakage cannot penetrate through the enclosing sheath. The surrounding body tissue is thus prevented from coming in contact with sharp edges. The resulting ceramic splinters remain inside the sheath and cannot be lost. Thus, on the whole, this yields an instrument that makes use of the high rigidity of the ceramic tube without having to accept its risks.
US 2005/0209618 A1 discloses a ceramic rod sheathed by an elastic tube, and US 2008/0300611 A1 discloses a metal-sheathed ceramic rod. Both of these designs reduce the harmful consequences of breaking ceramics due to a sheath which covers the breakage.
However, with the known designs, the ceramic rod extends essentially uniformly and with constant strength values over the length of the shaft. If an excessive load is applied to the ceramic, it may break at any location although its position cannot be predicted.
A laparoscopic instrument having a predetermined breaking point on a member that transmits a force is known per se from U.S. Pat. No. 5,389,104 A.